1. Field of the Invention
The present invention relates to the field of power boost circuitry for power supplies and, more particularly, to power boost circuits utilized in switching mode power supplies.
2. Prior Art
A number of power boost circuits are known in the applicable art. However, the present invention teaches a number of inventive advantages over such known circuits. Generally, it is an object of the present invention to provide a power boost circuit providing high speed and gain requiring minimal input power. It is a second object of the present invention to provide such a power boost circuit utilizing latching circuitry.
One such power boosting device is described in an article by E. Woodward entitled "Circuit switch AC load: the clean way, "EDN, p. 160-2, Nov. 20, 1975. The circuit described is slow to turn on. The time required to turn on the circuit is based on the constant: EQU .tau.=R.sub.2 .times.C.sub.2
Where R2 is a resistor and C2 is a capacitor. Capacitor C2 is a large capacitor, so the speed is correspondingly slow. Because the size of resistor R2 is fixed, the turn on speed of the "Darlington" is fully dependent on the value of capacitor C2, where the input current equals C2.times.(dv/dt). Besides the poor speed performance, this circuit cannot provide a permanent latch. The latching time is limited, where R1 is a resistor and C1 is a capacitor, to the constant: EQU .tau.=6.times.R.sub.1 .times.C.sub.1
Another booster circuit is described in note #120 of TRW Power Semiconductor by D. Roarke. This circuit had a capacitor coupled drive for the switching transistor. Control of the "Darlington" powder switch is provided by grounding the base of the transistor, or by supplying high input level of current. The speed of this circuit is limited by the speed of two transistors. Additionally this circuit is incapable of latching.
Another boosting circuit is described in U.S. Pat. No. 4,360,744 (Taylor), entitled "Semiconductor switching circuits". The Taylor semiconductor switching circuits are based on a cascaded connection. The cascaded connection has a low voltage, high current, high speed switching device connected in series with a high voltage, low speed transistor. The switching control of the Taylor circuit is that of a transistor which requires additional circuitry. This additional circuitry severely increases both the complication and expense of the circuit. Additionally, the Taylor circuit is limited in application.
The prior art circuitry depends on the switching speed of at least one transistor. Given this limitation, a circuit with a switching device that comprises a lesser number of current barriers will have an inherent speed advantage. What is needed is a high speed, high gain power booster circuit that is both capable of latching and requires relatively little initial driving power. One object of the present invention is to provide for such a circuit.